The present invention relates to a dispensing system for alkali metals capable of releasing a high quantity of these metals.
Alkali metals have been used in the field of electronics for a long time. In particular, these metals have been used in the past for the manufacturing of photosensitive surfaces, e.g. of image amplifiers or photomultiplier tubes, obtained by condensation of metal vapors onto inner walls of these devices.
Due to the high reactivity of alkali metals to atmospheric gases and to humidity, the evaporation sources generally used in industry are not the pure metals, but rather their compounds that are stable in air at room temperature in mixtures with reducing agents. The alkali metal compounds most commonly used are chromates, M2CrO4, but also used are molybdates, M2MoO4, tungstates, M2WO4, niobates, MNbO3, tantalates, MTaO3, titanates, M2TiO3, and similar salts (in these formulas M indicates any alkali metal); aluminum, silicon, or getter alloys, that is alloys based on titanium or zirconium with aluminum or one or more transition elements, are normally used as reducing agents. To favor the contact between the alkali metal compound and the reducing agent, both are preferably used in form of powders, with a particle size that is preferably smaller than 500 micrometers (μm); the weight ratio between the alkali metal compound and the reducing agent is generally comprised between 10:1 and 1:10. Mixtures of this type are described, for example, in British Patent GB 715,284, and U.S. Pat. Nos. 2,117,735; 3,578,834; 3,658,713; and 6,753,648, and in International patent application PCT/IT2005/000509.
These mixtures are normally used inside suitable dispensers capable of holding solid particles, but having at least a part of the surface permeable to alkali metal vapors, as shown for example in U.S. Pat. Nos. 3,578,834; 3,579,459; 3,598,384; 3,636,302; 3,663,121; and 4,233,936, and in Japanese patent application publication JP-A-4-259744. These dispensers are formed by a container, generally metallic, inside which there are loose powders or pellets of the dispensing mixtures. The heating of the mixtures is achieved by directly passing electric current through the walls of the dispenser, which then release heat by contact with the powders or pellets therein.
The dispensers disclosed in these patents are suitable for releasing small quantities of alkali metals, up to some tens of milligrams (see e.g., the dispenser described in Japanese patent application publication JP-A-4-259744, containing two pellets of the mixture). The release mode of these dispensers is such that once heated to the reaction temperature of the contained mixture, the alkali metal is completely released within a short time, whereupon the dispenser is exhausted. These operative characteristics are suitable for the formation of thin alkali metal layers in conventional applications, that is photomultipliers and image amplifiers.
Recently, the alkali metals, particularly lithium and cesium, have found a new application field in OLED screens (standing for “Organic Light Emitting Display”). Due to the importance of this application, reference is particularly made to this application in the following text, but the invention is of a more general applicability. The functioning principle of OLEDs is the recombination of electrons and electronic vacancies (or “holes”) inside a multilayer of different organic materials, interposed between two series of electrodes. For a detailed description of structure and operation of OLEDs reference can be made, for example, to European published patent applications EP-A-845924 and EP-A-949696; Japanese patent application publication JP-A-9-078058; and U.S. Pat. No. 6,013,384. The addition of small quantities of electron-donor metals, particularly alkali metals, to the structure of an OLED, allows reduction of the energy consumption of these screens. U.S. Pat. No. 6,013,384 describes the use of these metals as dopant for one or more layers of the organic multilayer, while U.S. Pat. No. 6,255,774 describes the use of these metals for the formation of very thin layers (less than 5 nanometers) between a series of electrodes (cathodes) and the adjacent organic layer. Both the formation of the organic multilayers and the addition of the alkali metals are obtained by evaporating the metals inside a deposition chamber, in which a substrate is placed being kept at such a temperature to allow the condensation of the vapors and the resulting formation of the desired thin layers.
In contrast to conventional applications, manufacturing on a very large scale is foreseen in the case of OLEDs, on the order of tens of million of pieces per year. To reach these numbers it is necessary to make use of continuous production (apart from short breaks for the cleaning of the chambers and the replacement of the evaporation sources). Also, the alkali metal dispensers must be able to operate for much longer times than was required in the past, for example in continuous cycles of about one week.
The prior art dispensers are not able to satisfy these requirements, while the simple increase of their dimensions has proved to be useless in practice.
In fact, with prior art dispensers, an increase of the dimensions with constant geometric shape would result in the decrease of the fraction of dispensing mixture in direct contact with the walls. Consequently, the thermal contact with the walls is good only for a fraction of the mixture that is smaller, the larger the dimensions of the dispenser, while the portions of the mixture further away from the walls receive heat only through the rest of the mixture, thus not efficiently due to the poor thermal conductivity properties of these mixtures.
Further, when using the mixtures described before, if an extension of the time of metal release is desired, it is necessary to progressively increase the temperature, in order to balance the reduction of the residual quantity of alkali metal in the container over time. By employing small dispensers, as those used up to now, this does not bring serious problems. Vice versa, increasing the dimensions of the dispensers, an increase in temperature of their walls implies a larger heat dissipation inside the process chamber. In these chambers control of the quantity of the deposited material is usually monitored during the process by devices called Quartz Crystal Microbalances (QCM) suitably positioned within the chamber. The actual sensor element of a QCM is composed of a quartz crystal, whose basic vibration frequency changes as a function of the weight of material deposited on it. By measuring the change of the vibration frequency with time with constant applied electric field, it is possible to determine the increase of weight of the material deposited on the crystal and, through the knowledge of the density of the material being deposited, the variation with time of the deposited thickness. An alkali dispenser of great size, by releasing considerable amounts of heat, can also heat up the quartz crystals by radiation, thereby increasing their temperature. Since the vibration frequency depends also on this latter parameter, there is an interference with the measurement of the deposit thickness, whereby precision in the process control is lost.